Polyester resins with excellent mechanical and chemical properties have been widely used in various applications of, for example, fibers for clothes, electromagnetic storages such as electromagnetic tapes and flexible disks, photographs, electrical insulation, cable lapping, capacitors, deposition, adhesive tapes, printer ribbons, magnetic cards, mold releasing for FRP, packaging, agricultures and so on.
By the way, from the viewpoint of guarding against fires, requests of allowing synthetic fibers or various plastic products to have flame resistance are increasing in recent years. However, conventional polyester resins still have plenty of room for improvement in the flame resistance. Due to this reason, it was proposed to add a flame retardant agent represented by a halogenated organic compound or an antimony compound at the time of polyester production.
However, there is a problem that these flame retardant agents exhale poisonous gases when contacting fire. To solve this problem, it was also proposed to add a hydrated metal compound such as aluminum hydroxide and magnesium hydroxide. However, it is needed to add a large amount of the hydrated metal compound to sufficiently improve the flame resistance. In this case, there is a fear of losing or deteriorating original excellent properties of polyester resins.
To solve these problems, it was proposed to add or copolymerize a phosphorus compound as the flame retardant agent at the time of polyester production. For example, Japanese Patent Early Publications [kokai] No. 6-16796, No. 2001-139784, and No. 2001-163962 disclose methods of copolymerizing a specific phosphorus compound with a polyester resin.
These phosphorus-containing polyester resins are often hard to dissolve in a general-purpose organic solvent such as toluene and xylene. Therefore, an extremely low polymerization degree is needed to obtain a solution or a dispersion liquid of the phosphorus-containing polyester resin used to treat PET films and fibers with the general-purpose organic solvent. However, in this case, there is a problem that it becomes difficult to keep the original properties of polyester resin.
Therefore, it is desired to develop a resin having the capability of exhibiting an improved applicability in the substrate treatment for fibers and PET films, while maintaining a high polymerization degree of the phosphorus-containing polyester resin to keep the original properties of polyester resin. To satisfy this requirement, when using an organic solvent with high solubility such as dioxane, DMF, HFIP and OCP, there are problems of poor working environment and environmental destruction. In addition, the use of these organic solvents induces another problem of deteriorating the substrates such as fibers and PET films. Since the phosphorus-containing polyester resin is not dispersed or dissolved in water, an aqueous solvent cannot be used as the solvent.